Showing papers by "Stephen W.S. McKeever published in 2006"

Radiation dosimetry for microbial experiments in the International Space Station using different etched track and luminescent detectors.

Abstract: The laboratory of Microbiology at SCKCEN, in collaboration with different universities, participates in several ESA programmes with bacterial experiments that are carried out in the International Space Station (ISS). The main objective of these programmes is to study the effects of space flight conditions such as microgravity and cosmic radiation on the general behaviour of model bacteria. To measure the radiation doses received by the bacteria, different detectors accompanied the microbiological experiments. The results obtained during two space flight missions are discussed. This dosimetry experiment was a collaboration between different institutes so that the doses could be estimated by different techniques. For measurement of the high linear energy transfer (LET) doses (>10 keV lm 1 ), two types of etched track detectors were used. The low LET part of the spectrum was measured by three types of thermoluminescent detectors ( 7 LiF:Mg,Ti; 7 LiF:Mg,Cu,P; Al2O3:C) and

Ionisation density dependence of the optically and thermally stimulated luminescence from Al2O3:C.

Abstract: This paper presents an overview of recent results on ionisation density dependence of the thermally stimulated luminescence (TL) and optically stimulated luminescence (OSL) signals from Al 2 O 3 :C, with emphasis on the sensitivity, efficiency, shape of the TL/OSL curves and the emission spectrum. High-ionisation densities are created uniformly by accumulated high doses of low-linear energy transfer radiation (gamma, beta, X rays) or non-uniformly in heavy charged particle tracks, even at low fluences, as in the case of space radiation fields. Significant deep trap filling, which occurs at these high-ionisation densities, ultimately results in changes in the concentration of recombination centres (F + -centres) and, consequently, in sensitivity changes and other effects. An OSL emission band at 335 nm has been observed in addition to the main F-centre luminescence band, and the relative intensities of these bands have been observed to be dependent on the ionisation density. The implications of these results and open issues are discussed.

The use of optically stimulated luminescence from AL2O3:C in the dosimetry of high-energy heavy charged particle fields

Abstract: This paper presents two different approaches of quantifying the optically stimulated luminescence (OSL) response of Al(2)O(3):C to high-energy heavy charged particles (HCPs). The OSL efficiency of Al(2)O(3):C exposed to different HCPs is defined as the sensitivity of the material to HCPs normalised by the sensitivity to gamma. In this paper, we investigate the possibility of introducing a 'mean efficiency' eta(mean), which when used in conjunction with the total gamma dose D(gamma) measured for a mixed radiation exposure allows for the determination of the absorbed dose without the need to determine the individual contributions of different types of radiation to the OSL signal. We tested the hypothesis that information regarding the 'mean efficiency' eta(mean) is contained in the shape of the OSL decay curve, using several approaches in the analysis of the OSL data. This analysis was applied to various mixed field irradiations performed at the HIMAC facility, Chiba, Japan. The results of this analysis are discussed.

A system to irradiate and measure luminescence at low temperatures.

Abstract: We have developed a system to irradiate samples and record radioluminescence (RL), optically stimulated luminescence (OSL), and thermoluminescence (TL) at temperatures ranging from -150 degrees C to 200 degrees C. The system consists of a cryostat, an irradiation/stimulation unit fitted with an X-ray tube (40 kV Moxtek) and a quartz window for optical stimulation, and a detection unit that utilises a photomultiplier tube and an interchangeable filter pack. Experiments have been conducted with quartz and albite (a feldspar). TL and OSL experiments show that several optically sensitive trapping states are stable below -50 degrees C. In addition, an increase in OSL is seen as the OSL stimulation temperature is lowered below -50 degrees C, and an increase in RL is apparent as the temperature is lowered during irradiation. This indicates that not only are optically sensitive low temperature traps present but that luminescence becomes more efficient at low temperatures.

Ionisation density dependence of the optically stimulated luminescence dose–response of AL2O3:C to low-energy charged particles

Abstract: The optically stimulated luminescence (OSL) response of Al 2 O 3 :C to high doses of gamma or beta irradiation can be used to predict the response of this material to charged particles as a function of particle fluence, particle energy and/or linear energy transfer (LET). In particular, it is predicted that track interaction effects at high particle fluences should result in linear-sublinear growth of the OSL signal. Similar considerations also predict a dependence of the fluence at which sublinearity starts upon the energy of the particles. In this work the OSL response of Al 2 O 3 :C to low-energy charged particles was investigated using protons (1, 2 and 4 MeV), carbon ions (13 MeV) and oxygen ions (10 MeV). The sublinear growth predicted above was qualitatively confirmed, but the energy dependence prediction was not. Furthermore, the efficiency of OSL production in the material after charged particle irradiation, compared to that obtained for gamma irradiation, is determined from the dose-response curves by fitting to a simple saturating exponential function. The efficiency values so obtained using this method are compared with those obtained from a conventional single-point measurement in the linear portion of the curve and found to be in good agreement. In general, the efficiency decreases as the LET of the particle increases. The present data are compared with published data obtained using high-energy charged particles and the results show that the efficiency is not a unique function of LET.

Progress towards a polymineral single-aliquot OSL dating procedure.

Abstract: Published single-aliquot regenerative-dose (SAR) procedures for quartz use a 'cutheat' different from the pre-heat. In contrast, previous work has shown that the SAR procedure can be used to correct for sensitivity changes exhibited by feldspars if the cutheat is equal to the pre-heat. In this paper, a procedure that corrects for sensitivity changes in both quartz and feldspar is tested. The results indicate that for sedimentary quartz the cutheat can be equal to the pre-heat. Thus, it may be possible to develop one procedure to measure equivalent doses in a polymineral sample. We also tested several different optical stimulation methods. Using the infrared (IR)-stimulated signal from a post-IR blue-stimulated optically stimulated luminescence sequence extends the potential age range when using feldpathic materials. In addition, since the post-IR blue-stimulated signal from feldspar is much smaller than the blue-stimulated signal from quartz, the possibility exists of extracting a 'quartz signal' from a mixed mineral sample.

Low temperature radioluminescence of LiF:Mg,Cu,P.

Abstract: Low temperature radioluminescence spectra of LiF, variously co-doped with Mg, Cu and P, show highly unusual temperature dependencies which resemble thermoluminescence data. The signals include intense peaks and a relatively weak continuous background. One peak occurs below 30 K, together with a major peak near 125 K. The signals are highly sensitive to the dopants and slightly sensitive to X-ray dose rate. The role of donor acceptor pairs and the perturbations from intrinsic defects formed by ionisation can be used to describe all the observations. The 290 nm emission band is linked to H center annealing.

In situ long-term monitoring system for radioactive contaminants

Abstract: A long-term in situ subsurface instrument for monitoring radioactive contaminant plumes, as an alternative to soil analysis, is described. A portable, laser-based reader optically stimulates luminescence from sensors, each containing an Al 2 O 3 :C dosemeter. The sensors, designed for placement at various subsurface locations around a waste site, are allowed to accumulate dose for a predetermined time that is based on the instrument's minimum detectable dose (MDD). The reader is then attached to the sensor by fibre optic cable to read the accumulated dose; an increase above natural background levels indicating the presence of leaked radioactivity. Based on an MDD of 5 μGy, it is shown that the sensor can measure soil concentrations of 1.85 Bq cm -3 after an exposure time of 50 h for 137 Cs and 67 h for 90 Sr/ 90 Y. Discrimination between beta and gamma radiation is possible using an end cap placed over one of the two paired sensors, allowing simultaneous measurement of 137 Cs and 90 Sr/ 90 Y in a mixed field. The monitor system represents a substantial improvement over quarterly soil sampling because of a greatly increased measurement frequency and the ability to perform measurements reproducibly.

Experimental determination of the dose deposition profile of a 90Sr beta source.

Abstract: Three different methods for characterising the dose deposition profile of a 90 Sr/ 90 Y radioactive source are described: GAFChromic film dosimetry, Thermoluminescence (TL) and Optically Stimulated Luminescence (OSL). For the film measurements, GAFChromic film samples were stacked at different depths between polyethylene terephthalate (PET) foils. For TL, the thickness of a TLD-500 dosemeter was gradually reduced by polishing and the TL from chips of different thickness was used in conjunction with a mathematical model based on the exponential attenuation of dose inside the crystal to determine the decay constant for the dose-depth profile. Finally, an OSL reader with confocal stimulation / detection capabilities was used to map the two-dimensional dose distribution in TLD-500 dosemeters as a function of depth. The shapes of the dose deposition profiles obtained from all the investigated methods are in good agreement.

TU‐E‐330D‐03: Evaluation of An OSL Dosimetry System for CT Quality Assurance and Dose Optimization

Abstract: Purpose: Recent development of wide‐beam multidetector CT and conebeam CT demands alternative methodology to CTDI. In this study, the Optically Stimulated Luminescence(OSL) technique was evaluated for CTquality assurance and dose optimization.Method and Materials:CT scans were performed on 38‐mm2 thin Luxel™ Al 2 O 3 : C dosimeters oriented in axial plane of a GE LightSpeed Ultra and at the center of a CT body dosimetry phantom. A 5‐mm beam collimation was selected to determine the energy response of the OSLdetector at 80, 120, and 140 kVp stations. mA response of the OSLdetector was evaluated from 60 to 350 mA. Helical scans of varying length coverage were also performed with the detector placed at the isocenter. The exposed OSL discs were measured using a Riso TL/OSL‐DA‐15 reader. Each OSL measurement was followed by a standard beta source irradiation and subsequent OSL measurement to normalize for the differing mass of the disks. The normalized OSL signal reading was used in the data analysis. Results: Good mA linearity was observed at all 3 kVp stations. There are discontinuities seen at ∼ 250 mA and it is known to be caused by tube focal spot change. The 120 and 140 kVp data show good correlation with ionization chamber reading. For 80 kVp, OSL signals to ion chamber readings are all higher, particularly at low mA range, indicating higher sensitivity of the OSLdetector over the ion chamber. The helical scans show an increased OSL signal with scan length and a leveling of the signal at large scan coverage. Conclusion: The OSLdetectors are of small size and responded well to CT exposure. It provides a practical technique for quantifying the dose at any location of a phantom for quality assurance and hence exhibits a potential for estimating patient organ dose.

SU‐FF‐I‐08: Results of An Optical Fiber‐Based Dosimetry System for Use in Computed Tomography Characterization

Abstract: Purpose: Modern multi‐detector CT and cone‐beam CT offer wide beams, making the concept of CT dose index (CDTI) no longer valid for CTdosimetry. A real‐time OSLdosimetrysystem has been developed and is evaluated for CTdosimetry in this study. Comparisons with a pencil ionization chamber were made. Methods and Materials: The system utilizes the optically stimulated luminescence(OSL) of KBr:Eu. The size of the KBr:Eu single crystal dosimeter equals approximately 1 mm3. The dosimeter was affixed to the terminal end of a plastic fiber cable and placed in the center hole of a plastic cylindrical phantom. The distal end of the fiber cable was attached to OSL reader, containing a 658 nm red laser, and photo‐multiplying tube (PMT), and associated optics/ electronics. CT slices of 1 s duration were performed over a range of energies (80–140 kVp) and tube currents (60–350 mA), as well as slice thickness (5 and 10 mm) using a GE LightSpeed Ultra scanner. Gantry tilt dependence was investigated over a range of 40.5° (22° superior to 18.5° inferior). OSL data was obtained before, during, and after the scan at the rate of 10Hz. Results: Performance was determined in part by normalizing both the initial OSL intensity and the background‐subtracted integral OSL to exposure reported previously by an ionization chamber. Good correlation between exposure and OSL data was found. Initial intensity and background‐subtracted OSL normalized to exposure show coefficients of variation of ∼%5 or less. Significant deviation was observed between the ∼10 OSL measurements taken for each slice, presumably as a result of absorption of x‐rays by the patient table. Conclusions: Initial tests have shown that this OSLdosimetrysystem possesses great potential for faster CT characterization. This system may prove a valuable alternative to CTDI.

Development of a luminescence dating device for in situ dating of geomorphological features on mars

Abstract: Introduction: The surface of Mars has been subject to eolian, fluvial, and periglacial activity. Unfortunately, establishing a chronology on Mars e.g. for dunes, gullies, the north polar layers, and other surface features is difficult. Consequently, techniques to quantify the ages of geomorphological processes on Mars have become an important area of research [1]. Among the techniques proposed is optically stimulated luminescence (OSL) dating, which is well established for age-dating sediments on Earth [2, 3]. We address some of the challenges associated with developing an OSL device for in situ dating of sediments on Mars. Results of experiments, using Martian simulant materials, as well as the design of an OSL instrument will be described. OSL Dating Principles: OSL Dating is a “dosimetric” technique. The time elapsed since deposition of a sediment layer is determined from the radiationdose accumulated in minerals since the last sunlight exposure, and the dose rate due to naturally occurring radioactive nuclides and cosmic radiation. Radiation exposure can be measured by stimulating the sample with light of one wavelength and monitoring the luminescence at another wavelength (OSL). The intensity of the OSL is a function of the absorbed natural radiation dose. If the rate of natural irradiation can be determined, then dividing absorbed dose by dose rate gives a radiation exposure age, i.e. the time elapsed since the last sunlight exposure. Challenges on Mars: An in situ OSL dating method for Martian surface sediments requires new measurement techniques in order to deal with the multiple new challenges not usually found when using OSL to date terrestrial sediments. Among the many challenges are: (1) the use of polymineralic samples, without the benefit of chemical separation, (2) ambient temperatures that are significantly lower than on Earth and highly variable, (3) a solar spectrum that is different from that on Earth, (4) a dose rate that is significantly higher than on Earth and dominated by high energy charged particles from galactic cosmic rays; the dose rate varies with depth, (5) anomalous fading of the OSL signal .......among several other challenges. Mineral composition. Using thermal emission data from the Mars Global Surveyor (MGS) Bandfield et al. [4] and Bandfield [5] identified areas on the Martian surface with mainly two different mineral compositions. The first one is composed of 65% plagioclase and 30 % clinopyroxene, and the second one of 45% plagioclase, only 10 % clinopyroxene and an additional 40 % potassium-rich glass. The plagioclase feldspars have calcium content of 30-70 % [6]. Based on these data we devised two mineral mixtures as surrogates for Martian sediments. The two mixtures are known as OSU-Mars-1 and OSU-Mars-2 and the compositions (in vol. %) are described in the following table: